1. Field of the Invention
The present invention relates to an image position matching apparatus and an image processing apparatus, and in particular to an image position matching apparatus for matching the positions of two images obtained of the same subject and an image processing apparatus for matching the positions of two images obtained of the same subject and performing image processes utilizing said two images of which the positions thereof have been matched.
2. Description of the Related Art
It is a common practice in a wide variety of fields to read and compare a plurality of two or more images obtained of the same subject in order to discern the difference between the images, and carry out an inspection of the subject based on the thus discerned difference, etc.
In the industrial products manufacturing industry, for example, images taken of a product when it is new and images taken of the same product after it has been subjected to an endurance test are compared, and attention is focused on the area showing the biggest difference so that an examination as to whether or not there are areas requiring improvement in product endurance can be carried out. In the medical field, doctors read and compare a plurality of radiation images of the diseased part of a patient taken in a temporal series in order to ascertain the course of the disease and determine the proper treatment.
In this fashion, reading and comparison of a plurality of images is carried out daily in a wide variety of fields. To facilitate such comparative reading, the two or more images are outputted by a display means, a printer or the like as visible images. Although the area of the most interest to the viewer of such images is the area wherein the greatest difference is shown, when the two or more images that are to become objects of comparative reading are compared, it is a common practice to simply line them up and display them. However, the ability to accurately discern the difference differs according to the skill level and experience of the operator, and under such conditions, the smaller the difference between the images is, the more difficult it becomes to discern. Therefore, there is a desire for an improvement in comparative reading performance, which is not dependent on the skill level of the diagnostician.
In this regard, interimage computation technology has already been proposed, as in, for example, Japanese Unexamined Patent Publication No. 11(1999)-342900, wherein: first, the anatomically characteristic positions between the two images to be comparatively read are coordinated; next, the coordinated positions are subjected to a subtraction process to extract the difference between said two images; and the difference is enhanced to obtain a subtraction image representing the difference between the two images. By extracting and enhancing only the interimage difference as described above, because the diagnostician is therefore enabled to accurately discern the difference between the two images, it can be said that the overlooking of a diseased portion or the progress of the treatment and the like can be prevented.
Further, when this interimage computation is to be performed, it is necessary to coordinate and match the positions of the structural components (anatomically characteristic positions) appearing within each of the two images. Regarding technology for performing the position matching operation, there are known two-step position matching technologies such as those proposed in U.S. Pat. No. 5,982,915 and Japanese Unexamined Patent Publication No. 7(1995)-37074), wherein, for example: a global position matching (e.g., a linear position matching employing an affine transform or the like) comprising the performance of parallel movement and/or rotation, or an enlargement and/or reduction process between two images is carried out; a plurality of regions of interest (template regions), which are small regions, are set in one of the two images subjected to the global position matching, and search regions, which are larger than each template region, are set in the other of the two images subjected to the global matching so that a search region is set for each corresponding template region; the template region and the image of the corresponding search region are substantially matched for each pair formed of a template region and corresponding search region; the portion region (the corresponding template region) within each search region is obtained; based on the positional relation between each template region of the one image and each corresponding template region of the other image, the shift quantity required to match each template region occurring in the one image to each corresponding template region in the other image is obtained; based on the obtained shift quantity, the aforementioned two globally matched images are subjected to a local matching process employing a nonlinear transform (warping) by use of a curve fitting process (a two-dimensional n polynomial function, where n>2); whereby the corresponding positions of the two images are matched to a comparatively favorable degree.
When images of the same subject are obtained at different times, there are cases in which there is fluctuation in the position of the body of the subject. In particular, when radiation images of the chest portion of a human subject are obtained at periodic intervals in a temporal series, there are cases in which the standing position, the posture, orientation, and the like of the patient will vary from one image to another, as a result, when two two-dimensional transparency images that have been obtained of the same subject at different times are compared, there are cases in which the bone tissue such as the ribs, and soft tissue, such as blood vessels, respiratory tract and the like appearing therein will have been displaced in different directions. Conventionally, when the original images of two two-dimensional transparency images (or reduction images of the original images, blurred images of the original images, etc.) between which the bone tissue and soft tissue are displaced in different directions were utilized in performing a global position matching process such as that descried above, an image wherein global position matching has been performed with respect to the soft tissue had been obtained.
However, when images of which the soft tissue positions thereof have been globally matched are utilized in performing a local matching process such as that described above, there are cases in which a rib appearing within a template region is not present in the corresponding search region. As a result, exceptionally large artifacts caused by positional misalignment occur within a subtraction image formed by a local matching process using the rib as the index, making it difficult to see the difference (diseased portion or the like) between the two images.
In response to this problem, there has been proposed in Japanese Patent Application Nos. 2001-44623 and 2001-129554 image position matching technology wherein images in which specific structural elements have been emphasized are utilized in a position matching process to match the positions of specified structural elements common to the images. If this technology is employed, by utilizing images in which the bone tissues have been emphasized, it becomes possible to obtain images of which the bone tissues have been globally matched, the number of artifacts appearing in a subtraction image obtained therebetween is reduced, and a diseased portion present in the connective tissue can be extracted with a high degree of accuracy.
However, if the positions of the bone tissues are matched between two two-dimensional transparency images in which the respective position of the body of the subject was different at the time of photographing, the accuracy with which the positions of the soft tissue are matched decreases, and it therefore becomes difficult to visually detect within the subtraction image the presence of a diseased portion in the soft tissue. That is to say, when there has been fluctuation in the position of the subject at the respective times of photographing, there is a limit to the degree to which the positions thereof can be matched, and it has proven difficult to match the positions of all structural elements common to the images.
On the other hand, along with an improvement in imaging technology, it has become possible to obtain three-dimensional transparency images, by use of a CT apparatus, an MRI apparatus or the like. Although there are cases in which it is desired to compare such a three-dimensional transparency image with a two-dimensional transparency image obtained in the past to observe the change over the course of time, if there has been fluctuation in the position of the body of the subject at the respective times of photographing, this comparison cannot be effectively performed. Further, from the standpoint of cost, it is advantageous to first obtain a three-dimensional transparency image and obtain for the second and subsequent images the comparatively less costly two-dimensional transparency image.